A monoclonal antibody, ST-4-39, was obtained by using a human gastric cancer xenograft, St-4, as an immunogen. Immunization was achieved by transferring immunocompetent mouse spleen cells into a nude mouse bearing St-4. Hybridomas were produced with the spleen cells of the mouse after rejection of the tumor and screened for immunohistochemical reactivity with cancers and normal tissues on formalin-fixed paraffin sections. ST-4-39 immunohistochemically reacted with various cancers including gastric, colorectal and pancreatic cancers as well as some normal tissues. ST-4-39 and NS 19-9 differed in immunohistochemical reactivity, although they reacted with some cancers and a few normal tissues in common. PBS extracts of normal and cancer tissues were examined for antigen reactive with ST-4-39 by sandwich enzyme immunoassay. Extractable antigen was detected in adenocarcinomas of colon, stomach and lung, while it was detected only in salivary gland and trachea among normal tissues examined. Gel filtration analysis of the antigen indicated a molecular weight of greater than or equal to 1 X 10(6), and the antigenic determinant was suggested to be a carbohydrate chain with terminal sialic acid by studies using periodic acid, neuraminidase and pronase treatments. Furthermore, the ST-4-39 antigen affinity-purified from two gastric cancer strains was shown to contain multiple carbohydrate determinants including sialyl-Lewisa and sialyl-Lewisx, suggesting the antigen to be a mucin.